feat(chat,todo): 小埃同学对话窗口与待办窗口

Chat:
- DTK2 窗口：左侧会话列表 + 右侧对话区 + 输入框
- DeepSeek / 自定义 API，SSE 流式输出，首字即显示
- 会话持久化到 chat.json，新建/切换/标题自动生成
- 设置对话框：API Key、API 地址、模型、系统提示，含开通指引
- 拖放：左侧空白=新建对话，左侧会话=指定会话，右侧对话区=当前会话
- Ctrl+Enter 发送，错误以消息形式回显

Todo:
- 待办窗口（layer-shell，不抢焦点、不进 dock），复选框/删除/新建
- 待办持久化 todo.json，--todo 打开，独立 desktop 入口

其他:
- desktop 入口改为打开对话窗口，--chat 参数
- 单实例消息支持 show/todo/chat

src/chat/chatclient.cpp

@@ -0,0 +1,182 @@
+// SPDX-FileCopyrightText: 2026 GXDE OS Team
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#include "chatclient.h"
+
+#include <QJsonArray>
+#include <QJsonDocument>
+#include <QJsonObject>
+#include <QNetworkReply>
+#include <QNetworkRequest>
+#include <QUrl>
+
+#include "config.h"
+
+ChatClient::ChatClient(QObject *parent)
+    : QObject(parent)
+{
+    m_manager.setRedirectPolicy(QNetworkRequest::NoLessSafeRedirectPolicy);
+}
+
+void ChatClient::send(const QVector<ChatMessage> &messages)
+{
+    cancel();
+    m_buffer.clear();
+    m_fullText.clear();
+    m_done = false;
+
+    const QString apiKey =
+        Config::instance()->get(QStringLiteral("chat/apiKey")).toString().trimmed();
+    if (apiKey.isEmpty()) {
+        Q_EMIT failed(tr("还没有配置 API Key。请先点右上角设置，填入 DeepSeek API Key。"));
+        return;
+    }
+
+    QString baseUrl = Config::instance()
+                          ->get(QStringLiteral("chat/baseUrl"),
+                                QStringLiteral("https://api.deepseek.com"))
+                          .toString()
+                          .trimmed();
+    while (baseUrl.endsWith(QLatin1Char('/'))) {
+        baseUrl.chop(1);
+    }
+    if (!baseUrl.endsWith(QStringLiteral("/chat/completions"))) {
+        baseUrl += QStringLiteral("/chat/completions");
+    }
+
+    const QString model = Config::instance()
+                              ->get(QStringLiteral("chat/model"),
+                                    QStringLiteral("deepseek-chat"))
+                              .toString();
+
+    QJsonArray jsonMessages;
+    for (const ChatMessage &message : messages) {
+        QJsonObject object;
+        object.insert(QStringLiteral("role"), message.role);
+        object.insert(QStringLiteral("content"), message.content);
+        jsonMessages.append(object);
+    }
+
+    QJsonObject body;
+    body.insert(QStringLiteral("model"), model);
+    body.insert(QStringLiteral("messages"), jsonMessages);
+    body.insert(QStringLiteral("stream"), true);
+    body.insert(QStringLiteral("temperature"), 0.7);
+
+    QNetworkRequest request{QUrl(baseUrl)};
+    request.setHeader(QNetworkRequest::ContentTypeHeader, QStringLiteral("application/json"));
+    request.setRawHeader("Authorization", "Bearer " + apiKey.toUtf8());
+    request.setAttribute(QNetworkRequest::RedirectPolicyAttribute,
+                         QNetworkRequest::NoLessSafeRedirectPolicy);
+
+    m_reply = m_manager.post(request, QJsonDocument(body).toJson(QJsonDocument::Compact));
+    connect(m_reply, &QNetworkReply::readyRead, this, &ChatClient::handleReadyRead);
+    connect(m_reply, &QNetworkReply::finished, this, &ChatClient::handleFinished);
+}
+
+void ChatClient::cancel()
+{
+    if (!m_reply) {
+        return;
+    }
+    QNetworkReply *reply = m_reply;
+    m_reply = nullptr;
+    reply->disconnect(this);
+    reply->abort();
+    reply->deleteLater();
+}
+
+void ChatClient::handleReadyRead()
+{
+    if (!m_reply) {
+        return;
+    }
+    m_buffer.append(m_reply->readAll());
+
+    int newline;
+    while ((newline = m_buffer.indexOf('\n')) >= 0) {
+        QByteArray line = m_buffer.left(newline);
+        m_buffer.remove(0, newline + 1);
+        if (line.endsWith('\r')) {
+            line.chop(1);
+        }
+        if (!line.startsWith("data:")) {
+            continue;
+        }
+        QByteArray payload = line.mid(5).trimmed();
+        if (payload == "[DONE]") {
+            m_done = true;
+            continue;
+        }
+        handlePayload(payload);
+    }
+}
+
+void ChatClient::handlePayload(const QByteArray &payload)
+{
+    const QJsonDocument document = QJsonDocument::fromJson(payload);
+    if (!document.isObject()) {
+        return;
+    }
+    const QJsonObject object = document.object();
+
+    if (object.contains(QStringLiteral("error"))) {
+        const QJsonObject error = object.value(QStringLiteral("error")).toObject();
+        Q_EMIT failed(error.value(QStringLiteral("message")).toString(tr("请求失败")));
+        return;
+    }
+
+    const QJsonArray choices = object.value(QStringLiteral("choices")).toArray();
+    if (choices.isEmpty()) {
+        return;
+    }
+    const QJsonObject deltaObject =
+        choices.first().toObject().value(QStringLiteral("delta")).toObject();
+    const QString content = deltaObject.value(QStringLiteral("content")).toString();
+    if (!content.isEmpty()) {
+        m_fullText += content;
+        Q_EMIT delta(content);
+    }
+}
+
+void ChatClient::handleFinished()
+{
+    if (!m_reply) {
+        return;
+    }
+    QNetworkReply *reply = m_reply;
+    m_reply = nullptr;
+
+    const int status = reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();
+    if (reply->error() != QNetworkReply::NoError && reply->error() != QNetworkReply::OperationCanceledError) {
+        const QByteArray body = reply->readAll();
+        QString message = reply->errorString();
+        const QJsonDocument document = QJsonDocument::fromJson(body);
+        if (document.isObject()) {
+            const QJsonObject error =
+                document.object().value(QStringLiteral("error")).toObject();
+            const QString detail = error.value(QStringLiteral("message")).toString();
+            if (!detail.isEmpty()) {
+                message = detail;
+            }
+        }
+        if (status == 401) {
+            message = tr("API Key 无效或已过期");
+        }
+        Q_EMIT failed(message);
+        reply->deleteLater();
+        return;
+    }
+
+    /* flush a final partial line */
+    if (!m_buffer.isEmpty()) {
+        QByteArray line = m_buffer.trimmed();
+        if (line.startsWith("data:")) {
+            handlePayload(line.mid(5).trimmed());
+        }
+        m_buffer.clear();
+    }
+
+    Q_EMIT finished(m_fullText);
+    reply->deleteLater();
+}